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Hemi is short for "hemisphere," the engine's iconic shape that is central to the performance that sets it apart from standard flathead engines. Flathead engines have greater surface area than a Hemi, which results in heat loss and lower efficiency. With Hemi engines, the high heat creates pressure in the cylinder, which burns fuel more efficiently and puts out more energy.

The 426 Hemi's narrow head also fits into smaller vehicles. The first three cars ever to have a 426 Hemi competed in the 1964 Daytona 500. Drivers who competed with cars that contained a 426 Hemi, such as Richard Petty, dominated that season. The Hemi was so successful that NASCAR amended its rules, banning the engine until Chrysler created a street version.

The position of valves and their size on the Hemi 426 improve airflow through the engine. Prior to Hemi, valve positioning was in-line, which meant valves had to be small. The valves on 426 Hemi are on the opposite sides of the engine's head, so they are larger. Better air flow means less wasted energy when pulling fuel and air into the combustion chamber and pushing out exhaust.